(University of Geneva/Denis Bajram)
The Hubble Space Telescope was the first to find direct evidence of an exoplanet's magnetosphere by tracking the distribution of carbon and hydrogen ions around it.
It turned out that warm Neptune HAT-P-11b has a plasmasphere and an extended tail of the magnetosphere.
Despite the fact that today several thousand exoplanets are known, for a long time it was not possible to directly study the properties of their magnetospheres.
It is assumed that the presence of a planet's magnetosphere may be indicated by radio emission of a certain nature emanating from it, modulation of the radiation of the stellar chromosphere in the Ca II line, or unusual features of exoplanet transits along the disk of its star.
In particular, ionized gas can be retained in the planet's plasmasphere (the inner region of the magnetosphere) and in the magnetosphere tail and change the transit time.
However, until now, all cases of observation could not be interpreted unambiguously.
Now a group of astronomers led by Lotfi Ben-Jaffel of the Institute of Astrophysics in Paris reported the first case of direct detection of a magnetosphere near an exoplanet.
The purpose of the observations was the warm Neptune HAT-P-11b, with a mass of 0.08 Jupiter's masses, located 123 light years from Earth in the orange dwarf system.
It was observed in the ultraviolet range by the Hubble using STIS and COS spectrographs; there were six cases of observations of exoplanet transits along the disk of its star in 2016-2017.
Scientists tracked the distribution of neutral hydrogen and oxygen, as well as singly ionized carbon near the exoplanet
Then, based on the observations, they built three-dimensional models of the magnetosphere.
It turned out that the atmosphere of HAT-P-11b should have a metallicity six times that of the Sun, and not 56, as it was determined in the course of earlier studies.
In addition, the researchers found that the exoplanet is surrounded by the plasmasphere and has an extended magnetosphere tail that holds ions and extends 1.8–3.1 astronomical units.
The equatorial magnetic field strength of HAT-P-11b is estimated at 1–5 gauss. For comparison, the strength of the Earth's magnetic field is only 0.25-0.65 gauss.
These properties of HAT-P-11b make the planet look more like a gas giant like Jupiter, rather than a Neptune-like planet.
Scientists intend to expand the modeling to the interior of the planet down to the core to include models of dynamo processes caused by convection, as well as to observe other ions in the HAT-P-11b magnetosphere.
Source:
- Nature Astronomy: https://www.nature.com/articles/s41550-021-01505-x
University of Arizona: https://news.arizona.edu/story/astronomers-detect-signature-magnetic-field-exoplanet